This invention relates to treatment of contaminated water and more particularly to the selective removal of trace concentrations of phosphates and chromates by means of a novel ion exchange material.
The presence of trace concentrations of dissolved phosphate is often responsible for eutrophication problems in lakes, reservoirs, other confined water bodies and coastal waters. Biological and physico-chemical treatment processes to remove phosphates from wastewater and other contaminated waters have been studied extensively. However, there has remained a major need for a viable fixed-bed process which can effectively eliminate phosphates. Previous investigations have shown the advantages as well as shortcomings of the fixed-bed process when strong-base anion exchangers, activated alumina and zirconium oxides are used as sorbents.
Trace amounts of phosphate (even less than one part per million) in treated wastewater from municipalities and industries are often responsible for eutrophication (excess nutrients and associated effects such as rapid growth of blue-green algae and hyacinth-like plants), which leads to short- and long-term environmental and aesthetic problems in lakes, coastal areas, and other confined water bodies. Concentrations less than 0.03 mg-P/L have been established as the criteria with regard to excessive algae growth in lakes and other confined water bodies in Switzerland (Gachter R. and Imboden D. M. (1985) Lake restoration. In Chemical Processes in Lakes. Stumm, W. (Editor), pp. 365-388. John Wiley & Sons, New York, N.Y.). A comprehensive investigation on the phosphate budget in Germany indicates that about 60 percent of the total phosphorus ending up in the aquatic ecosystem is contributed by domestic wastewater discharge ( Wolf P., et al. (1987) Stickstoff und Phosphor in Flie.beta.gewassern. Korrespondenz Abwasser, 34(11), 1215-1227. (In German) ). It is likely that a similar trend is being observed among other industrialized nations including the U.S. (Puckett L. J. (1995) Identifying the major sources of nutrient water pollution Envir. Sci. & Technol. 29(9) 408A-414A.). Very recently, the Water Environment Research Foundation prepared a detailed report that overviewed the present state of technology and identified high priority research needs pertaining to nutrient removal (Water Environment Research Foundation (1994): Research Needs for Nutrient Removal from Wastewater. Project 92-WAR-1).
Chemical Precipitation (CP) and Biological Nutrient Removal (BNR) are the two most commonly used methods for removal of phosphate from municipal and industrial wastewater (Jenkins D. and Hermanowicz S. W. (1991) Principles of chemical phosphate removal. In Phosphorus and Nitrogen Removal from Municipal Wastewater: Principles and Practice, Sedlak R. I. (Ed.); pp. 91-108. Lewis Publishers, 2nd ed., New York, N.Y.; Stensel H. D. (1991) Principles of biological phosphorus removal. In Phosphorus and Nitrogen Removal from Municipal Wastewater: Principles and Practice. Sedlak R. I. (Ed.), 2nd ed., pp. 141-163. Lewis Publishers, New York, N.Y.; Metcalf & Eddy (1991) Advanced wastewater treatment. In Wastewater Engineering: Treatment, Disposal, Reuse, Chapter 11. McGraw-Hill, Inc., New York, N.Y.). These processes essentially transfer phosphate from the liquid to the sludge phase which subsequently needs to be hauled and disposed of elsewhere. In general, CP and BNR processes are effective in reducing phosphate levels in municipal wastewater. However, these processes are sensitive to seasonal and diurnal variations in temperatures and changes in feed compositions. Also, complete removal is unattainable by CP and BNR due to thermodynamic and kinetic limitations.
In recent years, regulations on phosphate discharge have become particularly stringent in areas tributary to the Great Lakes, and semi-arid regions in the U.S. such as Arizona, Colorado, and Southern California, where wastewater reuse is in practice or being contemplated (Jenkins and Hermanowicz, 1991). In island resort areas where population and total phosphate discharge increase periodically, the use of CP and BNR processes for phosphate removal poses difficulties in adapting to fluctuating loads.
In the past, extensive research studies were also undertaken to explore the effectiveness of fixed-bed processes for phosphate removal because of their operational simplicity and adaptability to changing wastewater flowrates and compositions. Commercial anion exchangers, activated alumina, and zirconium oxides are among the well-studied sorbents. Their performances in regard to phosphate removal with varying degrees of success are well documented in the open literature (Boari G., Liberti L., and Passino R. (1976) Selective renovation of eutrophic wastes: phosphate removal. Wat. Res. 10, 421-428; Liberti L., Boari G. and Passino, R. (1976) Selective renovation of eutrophic wastes: phosphate/sulfateexchange. Wat. Res. 11 517-523; Lloyd L. and Dean R. B. (1970) Phosphorus removal from effluents in alumina columns. Jour. WPCF. 42(5) R161-172; Pollio F. X. and Kunin R. (1968) Tertiary treatment of municipal sewage effluents. Envir. Sci. & Technol. 2(1) 54-61; Urano K. and Tachikawa H. (1991) Process development for removal and recovery of phosphorus from wastewater by a new adsorbent. 2. Adsorption rates and breakthrough curves. Ind. Eng. Chem. Res. 30(8),1897-1899; Yoshida I. (1983) Studies on the selective adsorption of anion by metal-ion loaded ion-exchange resin. Separation Sci. & Technol. 18(1), 73-82). Some of the critical shortcomings with these sorbents can be summarized as follows:
(a) poor selectivity toward phosphate over other competing species, such as sulfate, chloride, bicarbonate, and dissolved organics; PA1 (b) very low capacity in the neutral pH range; PA1 (c) inefficient regeneration; and PA1 (d) gradual loss in capacity due to solution of the sorbent or fouling by organic matter.
Because of these limitations, WERF (1994) recognized the need to develop and identify a viable sorbent for phosphate removal from wastewater. The sorbent in the fixed-bed process should be durable and prefer phosphate over other dissolved competing species, namely, sulfate, chloride, nitrate, bicarbonate, and dissolved organic matter (DOM), which are commonly present in wastewater at much higher concentrations than phosphate. Also, in order to be cost effective, the sorbent should be amenable to efficient regeneration so that hundreds of cycles can be run for large-scale applications.